Field of the Invention
The present invention relates to a technology for diagnosing a deterioration of an exhaust gas purification apparatus disposed in an exhaust passage of an internal combustion engine, and in particular to a technology for diagnosing the deterioration of the exhaust gas purification apparatus equipped with a selective catalytic reduction (SCR) catalyst.
Description of the Related Art
As a method of diagnosing a deterioration of an exhaust gas purification apparatus disposed in an exhaust passage of an internal combustion engine, there has been known a method of using measured values of air fuel ratio sensors (or oxygen concentration sensors) which are disposed in a portion of the exhaust passage at the upstream side of the exhaust gas purification apparatus and in a portion of the exhaust passage at the downstream side of the exhaust gas purification device, respectively. Specifically, there has been known the technology in which an amount of oxygen (O2), which can be stored by the exhaust gas purification apparatus (hereinafter, referred to as an “oxygen storage capacity”), is obtained from a difference between outputs of the above-mentioned sensors disposed at two locations (hereinafter, referred to as a “sensor output difference”) at the time of changing the air fuel ratio of the exhaust gas flowing into the exhaust gas purification apparatus from a lean air fuel ratio higher than a stoichiometric air fuel ratio into a rich air fuel ratio lower than the stoichiometric air fuel ratio, and the deterioration of the exhaust gas purification apparatus is diagnosed based on the oxygen storage capacity thus obtained (see, for example, a first patent literature). In addition, in a second patent literature, there is disclosed a technology in which in an arrangement in which an NOx storage reduction (NSR) catalyst and an SCR catalyst are disposed in an exhaust passage of an internal combustion engine, the larger the degree of deterioration of the SCR catalyst, the lower the air fuel ratio of exhaust gas at the time of the execution of rich spike processing is made.